1. Field of the Invention
The invention relates to a method and arrangement for inserting a watermark in an information signal, more particularly, a high quality audio signal. The invention also relates to a method and arrangement for extracting the watermark from a watermarked signal.
2. Description of the Related Art
Watermarking is a well-known technique for protecting software against piracy. The general structure of a watermarking system is shown in FIG. 1. An original signal X is input to the system. A watermark WM is embedded in the signal X by a watermark embedder 100. The watermarked signal Y is perceptually indistinguishable from X but detectable by a detector 200. In practice, the watermarked signal Y is subjected to various types of signal processing P 300, for example, compression. If any type processing P renders the watermark WM undetectable from the processed signal Z, the watermark WM is said to be fragile. It is semi-fragile if some predefined signal-processing algorithms P leave the watermark WM detectable, whereas other types of processing render the watermark undetectable. The watermark WM is said to be robust if it can only be removed by very crude processing P, such that the reproduced signal is seriously degraded.
In the course of standardization of watermarking, the music industry has defined various types of processing P which allow discriminating between the above-mentioned types of watermarking. A number of existing watermarking schemes will be examined for compliance with the requirements set by the music industry. This relates both to sound quality and robustness.
The embedding of the watermark in current audio watermarking equipment is critically dependent on the audio signal representation. Existing watermarking schemes are optimized for signal representations with multiple bits per sample and sample rates not exceeding 96 kHz. A well-known example of such a signal representation is CD audio (16-bit PCM at 44.1 kHz). Other representations, such as high-rate signals, in which amplitude resolution is exchanged against time resolution, require different watermark embedding equipment or methods. An example of such a signal representation is Direct Stream Digital (DSD, 1-bit samples at 64xc3x9744.1 kHz) recorded on Super Audio CD.
It is an object of the invention to provide a universal method and arrangement for embedding and detecting a watermark in arbitrary-bit arbitrary-rate signals.
This object is achieved in a method of embedding a watermark in an information signal having a first sample frequency, the method comprising the steps sample rate converting said information signal to a predetermined second sample frequency; embedding the watermark in the converted signal at said second sample frequency; retrieving the watermark at the first sample frequency from the watermarked signal; and embedding the retrieved watermark in the information signal at the first sample frequency.
This object is also achieved in an arrangement for embedding a watermark in an information signal having a first sample frequency, the arrangement comprising a sample rate converter for converting said information signal to a predetermined second sample frequency; a watermark embedder for embedding the watermark in the converted signal at said second sample frequency; means for retrieving the watermark at the first sample frequency from the watermarked signal; and means for embedding the retrieved watermark in the information signal at the first sample frequency.
It is achieved with the invention that standard watermark embedders developed for multi-bit PCM signals can be used for any type of signal representation, even if the watermark is not directly available.